This invention relates generally to the field of injection molding of plastic articles, such as blowable plastic parisons. The parisons are typically formed by injecting molten plastic around a set of central core pins, and subsequently cooling the array of parisons while on the core pins. The cooled parisons next are stripped of the gate or tail portion of the parison and collected for subsequent blow molding into containers.
The art of container forming by blow molding a previously injection molded parison has advanced to the state where several thousand such containers can be formed per hour from a presupplied stock of parisons. In such a rapid production rate system, it is necessary to require that the parisons, from which the containers are ultimately blown, are themselves formed rapidly, inexpensively and with a high quality in each parison so that the total reject rate from the parison formation step is minimized. Accordingly in the injection molding process, which forms the parison for subsequently blow molding, it has become critical to reduce the overall cycle time of the injection molding cycle while increasing parison quality and reducing injection molding machine down time.
Currently available injection molding machines have not provided the overall desirable features disclosed above. For example, most currently available injection molding machines include a bipartate injection mold which terminates in flat space which houses an inlet port. An injection nozzle, which is connected to an injector assembly, which supplies molten thermoplastic material, fits into the inlet port with a very snug fit in the port. Also, current injection molding machines provide for a long tail or sprue section on the parison which must be removed prior to blow molding of the parison into a finished container. This provides for more waste plastic, which must be recylced, at a significant cost or discarded. Further, such an approach provides for high crystallinity in the parison sprue which is disadvantageous to the performance of the final blown container.
A number of problems are inherent in the above approach to injection molding of parisons. The large sprue or tail area on the bottom of the parison allows highly crystallizable materials, for example, polyethylene terephthalate, to crystallize as the parison is being cooled in the injection mold station. When the sprue is a crystallized plastic, it does not stretch well when the parison is being biaxially oriented to produce strong pressure resistant plastic containers such as those suitable for soft drink and beer packaging. Accordingly, the sprue must be removed in a subsequent station by a sprue removal device to provide a blowable parison for container fabrication. Such a step in parison fabrication requires additional cycle time thereby lowering the overall production of the injection molding machine. Further, in many cases, residual crystallinity remains in the sprue area of the bottom of the parison which is translated into a poorly oriented centrally located heel area on the bottom of the finished container. Such a poorly oriented heel area in the container is substantially less pressure resistant and provides an inferior container for holding pressurized fluids.
Further, when attempts were made to militate against the formation of highly crystalline sprue portions on parisons by moving the injection nozzle to intimate contact with the bottom of the parison, within the injection mold halves, further problems occurred. Namely, when the parison molds were opened, drooling or stringing of the injected plastic occurred which was detrimental to high quality parison production. A related, and very serious, problem occurred when the injection nozzle was placed such that the mold halves closed around the injection nozzle. Upon repeated clamping of the injection mold halves onto the hollow injection nozzle, as injection cycles occurred, the nozzles stress-cracked and were no longer useful. Further, at the end of the machine cycle and at the beginning of the cycle, as the molds heated and cooled they expanded and contracted. The nozzle also expanded and contracted, but usually at a different rate, since it was of a different metal alloy and in intimate thermal contact with the hot plasticizer. This situation led to crushing of the nozzle within the mold halves as differential thermal expansion and contraction occurred. This also caused premature replacement of the nozzles due to metal fatigue and subsequent failure.